This invention relates to arctic seismic exploration, and more particularly to a method and apparatus for generating seismic disturbances in the water below an ice layer, and recording seismic data in the form of reflective or diffractive signals generated in response to the seismic disturbances by sub-surface structural formations existing below the body of water.
Arctic exploration methods and equipment must overcome the problems of the ice-water terrain and frigid arctic temperatures. Since the ice most resembles a land terrain, heretofore methods of seismic exploration used on ice were similar to those of land exploration. This method involved using explosive charges in the form of point charges or a dispersed charge in the water beneath the ice depending on the distribution of energy desired. A recording truck towing a land cable having sections of geophones attached thereto is placed within an appropriate range to detect the seismic signal data created by the explosive charges. The two major problems with the land exploration method used on ice terrain are environmental and economic in nature. Environmentally, use of explosive charges in the water beneath the ice is found to be harmful to the water life therein. Economically, the use of explosive charges over other methods of generating seismic sources is far more expensive.
In marine seismic exploration, seismic events are created by detonating an explosive charge or by generating gaseous explosions using compressed air guns. There are also electrical discharge systems using an underwater spark to create an acoustic pluse, but these are mostly broadband and so generally inefficient with reference to narrow frequency bands of interest in seismic exploration. The seismic signal data generated by the seismic disturbance is detected by hydrophones attached to a streamer towed by a boat through the water.
Considering the terrain involved in arctic exploration, it is impossible to use a pure marine system. And, for the reasons described above, a pure land exploration system is not practical for arctic exploration. Although an air gun has been used in arctic regions to discharge air creating a seismic disturbance in the water beneath the ice and the seismic signal data from such disturbances has been recorded by geophones, this technique has been generally unreliable because of difficulties encountered in the operation of the air gun under the extreme conditions encountered in arctic regions. Also, air gun handling in both marine and primitive arctic methods heretofore employed has been manually accomplished, thus giving rise to a possible safety hazard and increasing the time cycle of exploration, which is defined by the time needed to create and record a seismic disturbance.